Currently, broadcasting satellites enable a plurality of wanted signals to be broadcast or transmission channels to be set in different transmitting spots in order to cover particular geographical zones. The channels are set, on the one hand, between the satellite and, on the other hand, terrestrial spots. The signals to be broadcast generally come from a terrestrial gateway, transmitting the data to be transmitted to the satellite via uplink channels.
FIG. 1 represents a configuration enabling a solution of prior art and that of the invention to be described. The satellite SAT includes a receiver for receiving signals from the gateway such as a receiving antenna and a processor for processing the received signals to be broadcast according to geographical zones. The satellite SAT is capable of generating a plurality of transmitting cones in order to cover a particular terrestrial zone, having in particular an interest in terms of population. The geometry of the transmitting cones is configured from a given antenna pointing. A transmitting cone is also called “beam” or coverage beam in literature. This is also called a transmitting spot insofar as a beam covers a spot corresponding to a geographical zone covered by the beam.
Such a broadcasting satellite SAT enables spots covering a complex geographical zone such as a part of the terrestrial surface that can form a line or curve and even a closed shape to be defined.
FIG. 2 represents about ten transmitting spots from a same satellite SAT. The spots Ci are numbered from C1 to C10.
A given geographical zone is covered by a transmitting spot, in particular such that terminals of this geographical zone can receive, demodulate and decode the base band signal.
The signals are transmitted in spots dedicated to a defined geographical zone. The latter are arranged so as to cover contiguous zones and define adjacent surfaces or cones. The satellite SAT makes it possible to affect to adjacent spots, frequency channels which are defined in different frequency bands and chosen so as to limit interference, intermodulation, or cross-modulation effects that can generate parasitic signals in neighbouring spots. However, by taking the satellite resources into account and optimising the bandwidth in each spot, the frequency used for a spot are reused for a spot not immediately neighbouring but adjacent to a neighbouring spot. Thus, according to the example of FIG. 2, spots C1, C3, C5, C7 and C9 share the same frequency band B1 and spots C2, C4, C6, C8 and C10 share the same frequency band B2 which is different from B1. But, a problem comes from the fact that transmissions dedicated to a given spot can cover, even if decreased, a zone close to the zone covered by the given spot.
Indeed, a problem arises from interference between two neighbouring spots but which are not directly adjacent, for example C2 and C4, the wanted signals of which transmitted in one of them can cause interference in the neighbouring spot and vice versa.
When a terminal is mobile, such as an aircraft, and that the latter switches from one zone to another, interference effects can occur at the terminal receiver. FIG. 3A illustrates a mobile terminal T1 switching from zone C4 to zone C3 towards zone C2. In this zone, the terminal T1 is more sensitive to the signals transmitted in spot C2 which can be received in spot C4, the coverage of which is noted S24. The signals are viewed by the terminal T1 as interference, noted S24. The terminal T1 can then undergo a significant degradation in the signals received by the receiving channel.
A problem exists in that the measured interference level can sometimes disturb the signals dedicated to a terminal when it is located in some positions of a spot likely to be exposed to interference due to the signals intended to neighbouring spots.